1. Field of the Invention
The present invention relates to wireless communication systems and, more particularly, to a method and apparatus for detecting messages transmitted over a communications channel such as a paging channel (PCH) defining part of a digital control channel (DCCH) in a cellular radio system.
2. Related Prior Art Systems
The prior art systems include cellular radio systems which have been operating in the United States since the early 1980s. In a typical cellular radio system as shown in FIG. 1, a geographical area (e.g., a metropolitan area) is divided into several smaller, contiguous radio coverage areas (called "cells") such as cells C1-C10. The cells C1-C10 are served by a corresponding group of fixed radio stations (called "base stations") B1-B10, each of which operates on a subset of the radio frequency (RF) channels assigned to the system. The RF channels allocated to any given cell may be reallocated to a distant cell in accordance with a frequency reuse pattern as is well known in the art. In each cell, at least one RF channel (called the "control" or "paging/access" channel) is used to carry control or supervisory messages, and the other RF channels (called the "voice" or "speech" channels) are used to carry voice conversations. The cellular telephone users (mobile subscribers) in the cells C1-C10 are provided with portable (hand-held), transportable (hand-carried) or mobile (car-mounted) telephone units (mobile stations ) such as mobile stations M1-M9, each of which communicates with a nearby base station. The base stations B1-B10 are connected to and controlled by a mobile services switching center (MSC) 20. The MSC 20, in turn, is connected to a central office (not shown in FIG. 1) in the landline (wireline) public switched telephone network (PSTN) or to a similar facility such as an integrated system digital network (ISDN). The MSC 20 switches calls between and among wireline and mobile subscribers, controls signalling to the mobile stations, compiles billing statistics, and provides for the operation, maintenance and testing of the system.
In the United States, two different entities are licensed to operate cellular systems in each major metropolitan statistical area (MSA). To receive service, a mobile subscriber enters into a subscription agreement with one of these local operators (the local system from which service is subscribed is called the "home" system). When travelling outside the home system (called "roaming"), a mobile subscriber may be able to obtain service in a distant (called "visited") system if there is a roaming agreement between the operators of the home and visited systems. Access to a cellular system by any of the mobile stations M1-M9 is controlled on the basis of a mobile identification number (MIN) which is assigned to each mobile subscriber by the home system operator, and the electronic serial number (ESN) which is permanently stored in the mobile station (the so-called "MIN/ESN pair"). The MIN/ESN pair is sent from the mobile station when originating a call and its validity is checked by the MSC 20. If the MIN/ESN pair is determined to be invalid, the system may deny access to the mobile station. The MIN is also sent from the system to the mobile station when alerting the mobile station of an incoming call.
When turned on (powered up), each of the mobile stations M1-M9 enters the idle state (standby mode) and tunes to and continuously monitors the strongest control channel (generally, the control channel of the cell in which the mobile station is located at that moment). When moving between cells while in the idle state, the mobile station will eventually "lose" radio connection on the control channel of the "old" cell and tune to the control channel of the "new" cell. The initial tuning to, and the change of, control channel are both accomplished automatically by scanning all the control channels in operation in the cellular system to find the "best" control channel. When a control channel with good reception quality is found, the mobile station remains tuned to this channel until the quality deteriorates again. In this manner, the mobile station remains "in touch" with the system and may receive or initiate a telephone call through one of the base stations B1-B10 which is connected to the MSC 20.
To detect incoming calls, the mobile station continuously monitors the control channel to determine whether a page message addressed to it (i.e., containing its MIN) has been received. A page message will be sent to the mobile station, for example, when an ordinary (landline) subscriber calls the mobile subscriber. The call is directed from the PSTN to the MSC 20 where the dialed number is analyzed. If the dialed number is validated, the MSC 20 requests some or all of the base stations B1-B10 to page the called mobile station throughout their corresponding cells C1-C10. Each of the base stations B1-B10 which receive the request from the MSC 20 will then transmit over the control channel of the corresponding cell a page message containing the MIN of the called mobile station. Each of the idle mobile stations M1-M9 which is present in that cell will compare the MIN in the page message received over the control channel with the MIN stored in the mobile station. The called mobile station with the matching MIN will automatically transmit a page response over the control channel to the base station which then forwards the page response to the MSC 20. Upon receiving the page response, the MSC 20 selects an available voice channel in the cell from which the page response was received (the MSC 20 maintains an idle channel list for this purpose), and requests the base station in that cell to order the mobile station via the control channel to tune to the selected voice channel. A through-connection is established once the mobile station has tuned to the selected voice channel.
When, on the other hand, a mobile subscriber initiates a call (e.g., by dialing the telephone number of an ordinary subscriber and pressing the "send" button on the telephone handset in the mobile station), the dialed number and MIN/ESN pair for the mobile station are sent over the control channel to the base station and forwarded to the MSC 20 which validates the mobile station, assigns a voice channel and establishes a through-connection for the conversation as described before.
If the mobile station moves between cells while in the conversation state, the MSC 20 will perform a "handoff" of the call from the old base station to the new base station. The MSC 20 selects an available voice channel in the new cell and then orders the old base station to send to the mobile station on the current voice channel in the old cell a handoff message which informs the mobile station to tune to the selected voice channel in the new cell. The handoff message is sent in a "blank and burst" mode which causes a short but hardly noticeable break in the conversation. Upon receipt of the handoff message, the mobile station tunes to the new voice channel and a through-connection is established by the MSC 20 via the new cell. The old voice channel in the old cell is marked idle in the MSC 20 and may be used for another conversation.
The original cellular radio systems, as described above, used analog transmission methods, specifically frequency modulation (FM), and duplex RF channels in accordance with the Advanced Mobile Phone Service (AMPS) standard. According to the AMPS standard, each control or voice channel between the base station and the mobile station consists of a pair of separate frequencies, a forward (downlink) frequency for transmission by the base station (reception by the mobile station) and a reverse (uplink) frequency for transmission by the mobile station (reception by the base station). The AMPS system, therefore, is a single-channel-per-carrier (SCPC) system allowing for only one voice circuit (telephone conversation) per RF channel. Different users are provided access to the same set of RF channels with each user being assigned a different RF channel (pair of frequencies) in a technique known as frequency division multiple access (FDMA). This AMPS (analog) architecture was the basis for the industry standard sponsored by the Electronics Industries Association (EIA) and the Telecommunication Industry Association (TIA), and known as EIA/TIA-553.
More recently, however, the cellular industry began migrating from analog to digital technology, motivated in large part by the need to address the growth in the subscriber population and the increasing demands on system capacity. To this end, the EIA/TIA has developed two distinct series of digital standards, both of which rely on voice encoding (digitization and compression) to multiply the number of voice circuits (conversations) per RF channel (i.e., to increase capacity), but each using a different access method. One of the EIA/TIA standard series uses code division multiple access (CDMA). The current standard in this series is known as the IS-95 standard. The other EIA/TIA standard series uses time division multiple access (TDMA). The original standard in this other series was known as the IS-54 standard. To ease the transition from analog to digital and to allow the continued use of existing analog mobile stations, the IS-54 standard supported the original AMPS analog voice and control channels and additionally provided for the use of digital traffic channels for speech (but not digital control channels) within the existing AMPS network. As a result, the IS-54B (the current revision of the IS-54) standard has also become known as the digital AMPS (D-AMPS) standard. However, the industry has now developed a new specification for D-AMPS which includes a digital control channel. This new specification is known as IS-136 and it builds on the IS-54B standard. All of the foregoing standards are hereby incorporated herein by reference (copies of these standards may be obtained from the Electronics Industries Association; 2001 Pennsylvania Avenue, N.W.; Washington, D.C. 20006).
According to IS-54B and as shown in FIG. 2, each RF channel is time division multiplexed (TDM) into a series of repeating time slots which are grouped into frames carrying from three to six digital speech channels (three to six telephone conversations) depending on the source rate of the speech coder used for each digital speech channel. Each frame on the RF channel comprises six equally sized time slots (1-6) and is 40 ms long (i.e, there are 25 frames per second). The speech coder for each digital traffic channel (DTCH) can operate at either full-rate or half-rate. A full-rate DTCH uses two equally spaced slots of the frame (i.e., slots 1 and 4, or slots 2 and 5, or slots 3 and 6). When operating at full-rate, the RF channel may be assigned to three users (A-C). Thus, for example, user A is assigned to slots 1 and 4, user B is assigned to slots 2 and 5, and user C is assigned to slots 3 and 6 of the frame as shown in FIG. 2. Each half-rate DTCH uses only one time slot of the frame. At half-rate, the RF channel may be assigned to six users (A-F) with each user being assigned to one of the six slots of the frame as also shown in FIG. 2. Note that although IS-54B, for convenience, defines a "frame" to be 40 ms long for both half-rate and full-rate channels, in reality the "TDM frame" is 40 ms long for half-rate channels, but it is only 20 ms long for full-rate channels (where a "TDM frame" is defined as the minimum amount of time between two repeating time slots in the same channel assigned to the same user).
Hence, unlike an analog FDMA cellular system in which the base station and the mobile station transmit and receive continuously over an RF channel, a TDMA cellular system operates in a buffer and burst dis-continuous transmission mode. Each mobile station transmits and receives in an assigned slot on the RF channel. The transmit (reverse) and receive (forward) frames, however, are offset from each other by at least one time slot period so that the mobile station will transmit and receive at different times and, therefore, can avoid using duplexer circuitry (which would be needed if the transmitter and receiver at the mobile station operated simultaneously). Thus, slot 1 of frame N in the forward direction occurs at least one time slot period after slot 1 of frame N in the reverse direction. At full-rate, for example, a mobile station assigned to slot 1 (user A in FIG. 2) would transmit for one slot period, receive in the next slot period, hold for another slot period, and then repeat this cycle. The mobile station, therefore, transmits or receives in a fraction of the time (one third for full rate and one sixth for half-rate) and can be switched off to save power the rest of the time.
Similar power savings can be achieved in the mobile station if digital transmission techniques are also applied to the control channel. The original analog control channel specified in EIA/TIA-553 and imported into IS-54B (because of the need to serve existing analog-only mobile stations) carried an overhead message train (OMT) which required continuous monitoring by the mobile station. Furthermore, an idle mobile station listening to the forward control channel was required to read all the messages transmitted in the OMT (not just paging messages) even though the information contained in these messages may not have changed from one OMT to the next OMT. These two requirements tend to unnecessarily limit the mobile station battery life. To overcome these requirements, U.S. Pat. No. 5,404,355 to Raith (the present inventor) suggests the use of a digital control channel (DCCH) which may be defined alongside the digital traffic channels (DTCH) specified in IS-54B. Referring back to FIG. 2, a half-rate DCCH would occupy one slot while a full-rate DCCH would occupy two slots out of the six slots in each 40 ms frame. The DCCH slots may then be organized into a series of superframes each comprising a plurality of logical channels which carry different kinds of information with each logical channel being allocated one or more DCCH slots.
FIG. 3 shows an exemplary DCCH superframe which includes at least three logical channels, namely, a broadcast control channel (BCCH), a paging channel (PCH), and an access response channel (ARCH). The BCCH, which in this example is allocated 6 DCCH slots, carries overhead messages. The PCH, which is allocated one DCCH slot, carries paging messages. The ARCH, which is also allocated one DCCH slot, carries voice or speech channel assignment messages. The exemplary superframe of FIG. 6 may contain other logical channels, including additional paging channels (if more than one PCH is defined, different groups of mobile stations may be assigned to different PCHs). A mobile station operating on the DCCH of FIG. 3 need only be "awake" (monitoring) during certain time slots (e.g., the BCCH and its assigned PCH) in each superframe and can enter "sleep mode" at all other times. While in sleep mode, the mobile station turns off most internal circuitry and saves battery power. Furthermore, by configuring the BCCH as taught in the aforementioned U.S. Pat. No. 5,404,355, the mobile station can read (i.e., decode) the overhead messages when locking onto the DCC (e.g., at power-up) and thereafter only when the information has changed, thus resulting in additional battery power savings while allowing for fast cell selection.
For maximum sleep mode efficiency, however, it is also desirable that the mobile station avoid the requirement of reading all PCH messages since only a fraction of the page messages received over the PCH will be directed to the mobile station and the other messages will either be empty messages ("filler" messages containing no page) or pages to other mobile stations. In practice, most of the messages carried over the PCH will be empty page messages. This is due to the fact that the PCH will usually be operated substantially below the capacity limit in order to avoid excessive traffic blocking (and, hence, delay in delivering pages to the mobile stations). If blocking problems do develop (e.g., because of unanticipated demand) in any cell, the operator can assign additional control channels in that cell or use other capacity-enhancing techniques such as cell splitting. Thus, in general, an appropriately-managed PCH will be operated at a level far below maximum capacity, even at busy hour. Viewed over a 24-hour period, including hours of low traffic activity, the average traffic carried on the PCH will be significantly less than the maximum capacity. Consequently, more often than not, the PCH is carrying not page messages but empty messages. Furthermore, since a mobile station usually receives no more than a few calls each day, most of the page messages sent on the PCH will be for other mobile stations.
To maximize sleep mode efficiency, the mobile station should be able to detect whether the received page messages are relevant messages (e.g., page messages directed to this particular mobile station) or irrelevant messages (e.g., empty page messages or page messages directed to other mobile stations) as early as possible in the receive processing (e.g., after demodulation but before decoding) so as to avoid as many signal processing steps as possible. Once an irrelevant page is detected, the mobile station can immediately return to sleep. To appreciate the possible power savings from an early detection of irrelevant pages, consider a typical PCH in which a paging message is sent once per second. This means that there are (60*60*24=) 86,400 page messages sent to the mobile station each day. If, for example, the PCH carries non-empty page messages only 10% of the time, the mobile station can avoid processing 90% of the page messages if it can detect empty pages. Furthermore, if only a few of the non-empty page messages are directed to this mobile station, it can avoid processing almost all of the page messages transmitted on the PCH if it can also detect that the other non-empty page messages are directed to other mobile stations. Thus, the mobile station effectively can be in sleep mode during PCH reception.